<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <button>点击</button>
</body>
<script>
    let btn = document.querySelector('button');
    //事件监听不会被覆盖
    btn.addEventListener('click', function(){
        alert(11);
    });
    btn.addEventListener('click', function(){
        alert(22);
    })
</script>
</html>